I. Field of the Invention
The present invention relates to the field of microfabricated transducers. More specifically, the present invention relates to microfabricated transducers formed on the surface of a substrate and a method of packaging and isolating such transducers.
II. Description of the Related Art
Microfabricated transducers are devices made with the techniques of the semiconductor industry such as lithography, chemical vapor deposition, plasma etching, wet chemical etching and many others. These devices contain structures capable of converting energy from the electrical domain to another physical domain. Examples of other physical domains include but are not limited to the acoustic, chemical, and optical domains. Transducers can also convert energy from said physical domains into an electrical signal. Surface microfabricated transducers describe a subset of microfabricated transducers that are formed on and whose entire function is contained within the surface portion of the supporting substrate, typically a silicon wafer. The surface portion is typically considered to represent up to 2% of the thickness of the substrate (0.1-10microns for a typical 500 micron silicon wafer).
One example of a surface microfabricated transducer is the acoustic transducer disclosed in U.S. patent application Ser. No. 09/315,896 filed on May 20, 1999 entitled “ACOUSTIC TRANSDUCER AND METHOD OF MAKING THE SAME” and assigned to the same assignee as the present application. In operation, such a transducer, as shown in FIG. 1, can be used to generate an acoustic signal or to detect an acoustic signal. By generating electrical signals on the electrodes of the transducer, an electrostatic attraction between the electrodes 16 and 18 is caused. This attraction causes oscillation of the membrane 14, which, by thus moving, generates the acoustic signal. Similarly, an incoming acoustic signal will cause the membrane 14 to oscillate. This oscillation causes the distance between the two electrodes 16 and 18 to change, and there will be an associated change in the capacitance between the two electrodes 16 and 18. The motion of the membrane 14 and, therefore, the incoming acoustic signal can thus be detected. Arrays of acoustic transducers, whether integrated with electronics or not, are also known. In a typical acoustic transducer array, independent acoustic transducers are capable of being excited and interrogated at different phases, which enables the imaging functionality.
Because transducers convert energy between the electrical and another domain, they need to be in physical contact with the domain of interest. An acoustic transducer, for example, needs to be exposed to the medium in which it is to launch and receive acoustic waves. A chemical sensor measuring concentration, such as a humidity sensor, needs to be exposed to the environment in which it is trying to measure humidity. An optical sensor, measuring light, needs a transparent window to provide exposure to the optical environment. Thus, the packaging of microfabricated transducers must provide not only electrical connections and protection to the transducer, but also environmental exposure. Such complicated packaging can in many instances be more costly than the fabrication of the transducers themselves.
Therefore, a packaging methodology that takes advantage of the techniques used in transducer fabrication (sequences of film depositions, lithographic pattern definitions, and selective removal of film material) to reduce the cost of transducer packaging is highly desirable. Furthermore, in cases where many transducer elements are operated in an array configuration, such as in ultrasonic transducer arrays, droplet ejector arrays, etc, it may be desirable for the packaging to help isolate one element from the others. The packaging can help to mechanically or electrically isolate the elements. Further still, the packaging may be flexible, such as flex circuits known in the art, and in this manner enable flexible transducer arrays capable of adopting curved configurations.
It has recognized by the present inventor that the relatively flat topology of surface microfabricated devices allows them to be packaged with many of the techniques and materials of the printed circuit board industry. The present inventor has further recognized that in the specific case of microfabricated ultrasonic transducers, cross-coupling between array elements could be problematic. Cross-coupling can occur electrically or acoustically. While special precautions can be taken during transducer and substrate preparation to reduce or eliminate electrical and acoustic cross-coupling through the substrate, a particular interface wave known as the Stonely wave is responsible for much of the cross coupling observed in microfabricated ultrasonic transducer arrays. This wave propagates in parallel to the interface of two materials. Because microfabricated ultrasonic transducers tend to have a displacement component in this direction, as shown in FIGS. 2A and 2B, Stonely waves may be launched at the edges of array elements.
What is needed therefore, is a method of packaging surface microfabricated transducers which provides protection and electrical connections to the transducer, exposes the transducer to the medium of interest, and isolates the transducer from neighboring elements when relevant.